When a potato cooks, the starches contained in each cell are released as the cell walls break down. These starches absorb the potato’s internal moisture and swell and soften. These two processes are what transform a raw, hard potato into a cooked, softer potato fit for mashing.
If you cut your potatoes up before boiling them, the starches absorb the internal potato moisture as well as the water in which they’re being boiled. If they boil for too long, they absorb too much water and your mashed potatoes will become gummy. The difference between perfectly cooked potato pieces and soggy pieces can sometimes be as little as a minute or two.
An easy workaround is to boil potatoes whole. They’ll take a bit longer to cook but you can leave them in the hot water after boiling without undesirable effects, keeping them warm until it’s time to mash them. I start my potatoes boiling as soon as I begin cooking and mash them immediately before dinner.
If they need more moisture, you can add a bit of hot potato boiling water or another liquid. This way, you have more control of their moisture content.
You are correct, op has no idea what they are talking about. Potatoes contain two different types of starch, in differing amounts depending on the kind of potato. Amylase, and Amylopectin, the second when overworked will make gummy potatoes. Overworking them is what makes them gummy.
1" dice, fork tender, food mill, ricer, or fork, perfect. If you want them extra smooth simmer in milk or cream (not water), strain and mash, then fold in the drained liquid gently with a spatula.
And the ‘hot potato boiling water’ contains mostly Amylase. If you add that it will make them more slippery/slimey.